1. Field of the Invention
The present invention relates to systems for regulating the contraction of a heart and more particularly to defibrillator and demand pacer catheters.
2. Background Art
The heart is a large hollow muscle used to pump blood to all parts of the body. Veins bring blood to the heart while arteries carry blood away from the heart. Valves control the flow of blood through the heart. The function of blood is to carry oxygen to the body parts. If the heart stops, the oxygen is no longer delivered and the body dies.
A muscle wall known as the septum divides the heart lengthwise into a right side and a left side. Each side has two chambers, one above the other. The upper chamber on the right side is the right atrium. The right atrium collects blood from the body through the veins. When the right atrium is full, a valve known as the tricuspid valve opens, providing a passage to the lower chamber. The muscle tissue surrounding the right atrium then contracts, pushing the blood into the lower chamber.
The lower chamber is called the right ventricle. Once the tricuspid valve closes, the muscle tissue surrounding the right ventricle contracts, pushing the blood to the lungs. The lungs provide oxygen that is absorbed into the blood. The blood then flows to the left side of the heart where it passes through a left atrium and a left ventricle, finally being ejected from the left ventricle to circulate through the body.
The heart has a special system of muscles that cause the heart tissue to regularly and continuously contract. One part of the system called the S-A node regularly emits electrical signals or pulses that travel through the heart tissue to a second point in the heart called the A-V node. The heart tissue contracts in response to the electrical pulses. A second part of the system called the bundle of His regulates the electrical pulses. The bundle of His insures that all muscle tissue surrounding a specific compartment simultaneously contracts and that the atriums and ventricles contract at the appropriate time. One complete contraction of both the ventricles and the atriums constitutes a beat.
On occasion, the electrical pulses being carried through the heart tissue become irregular, causing the heart to beat rapidly or unevenly. Defibrillation is a process used to restore normal beating to a heart in this condition. To defibrillate a heart, a large electrical charge called an electrical defibrillation pulse is applied directly to the heart. This electrical defibrillation pulse works to depolarize the electrical pulses of the heart and, thereby, restore normal beating.
There are other occasions in which the heart fails to deliver a specific electrical pulse, thereby causing the heart to pause, skipping a beat. Demand pacing is a process used to maintain normal beating of a heart in this condition. To demand pace a heart, sensors are used to determine if the heart is delivering its electrical pulses at the appropriate time. If the heart is not, a relatively small electrical charge called an electrical demand pacing pulse is applied directly to the heart to assist in electrical depolarization of the heart tissue. Individuals having heart problems often suffer from both of the above conditions and, thus, are benefited if they can receive both defibrillation and demand pacer pulses.
The electrical pulses for defibrillation and demand pacing are typically delivered to the heart through different methods. The defibrillation pulse has historically been delivered through a large area electrical patch sewn to the exterior surface of the heart. The electrical patch is connected to a capacitor that is charged by a battery, thereby to be capable of delivering an electrical defibrillation pulse. Once the capacitor discharges the defibrillation pulse, it enters directly into the heart of the patient so as to defibrillate the heart. The pulse then exits the body through a ground electrode attached to the skin of the patient.
Attaching the electrical patch to the heart of the patient requires an extensive operation in which the rib cage is separated so as to expose the heart. Once the heart is exposed, the patch can be sewn to the exterior surface. The surface area of the electrical patch must be of a sufficient size to deliver the high energy defibrillation pulse without burning the tissue of the heart. A typical electrical patch has a surface area in the range of about 50 cm.sup.2 to about 100 cm.sup.2, and is capable of delivering a defibrillation pulse with an energy of up to about 50 joules.
In contrast, the demand pacing pulses are often applied to the heart by a demand pacer catheter. The demand pacer catheter is a long flexible probe, usually made of stilastic or polyurethane, with electrical leads running the length of the catheter at the middle thereof. At one end of the probe, the leads are connected to an exposed metal surface called a demand pacer electrode. Part way up the probe, the leads are connected to a second exposed metal surface called a ground electrode. Finally, at the other end of the probe, the leads are connected to a regulator that has a controller for sensing the beat of the heart and a capacitor charged by a battery, for sending the demand pacer pulses to the heart.
The demand pacer catheter is used by making an incision in a vein leading to the heart. The end of the probe with the demand pacer electrode is inserted into the vein and threaded to the heart and into the right ventricle. When the heart delivers its electrical pulse to the muscle tissue, the signal is carried up the lead wires in the probe and to the controller. If the heart fails to deliver its electrical pulse, the controller senses the missing signal and tells the capacitor to transmit the electrical demand pacer pulse to the demand pacer electrode. Once emitted from the electrode, the pulse travels through the blood in the right atrium and into the surrounding heart tissue, thereby causing depolarization of the heart. The pulse finally leaves the body through the ground electrode.
In one version of the demand pacer catheter, the demand pacer electrode has a helical or corkscrew-shaped electrode for attachment to the interior of the heart. Such corkscrew-shaped electrodes typically have a length of about 5 mm with a surface area of about 6 mm.sup.2. Furthermore, such electrodes are only capable of delivering an electrical current of about 5 milliamperes.
The trouble with the above-discussed approaches for applying the different electrical pulses is that two procedures are required: One for inserting the demand pacer catheter and one for attaching the defibrillator electrical patch. Furthermore, attaching the electrical patch is an extensive operation, exposing the patient to high risk conditions and requiring a long recovery period.
Attempts have been made to solve the above problems by producing a single catheter that can be inserted into the heart for applying both defibrillation and demand pacer pulses. One such catheter is an implantable, self-contained system for sensing the pulse of a heart and for automatically sending a defibrillator or demand pacer pulse to the heart depending on the condition of the heart. An example of such a catheter is found in U.S. Pat. No. 3,857,398 issued to Leo Rubin of the present invention.
Similar to the demand pacer electrode previously discussed, this catheter has a flexible probe that can be inserted into a vein and threaded through the right atrium and into the right ventricle of the heart. A ground electrode and a demand pacer electrode are attached to the portion of the probe in the right ventricle. A defibrillator electrode is attached to the portion of the probe in the right atrium. Connected to the other end of the probe is a regulator having a controller for sensing and analyzing the electrical pulse of the heart. The regulator further includes a defibrillator capacitor and demand pacer capacitor for transmitting their respective pulses to the heart. The capacitors are charged by a battery also located in the regulator. The regulator is inserted into the body, such as in the subcutaneous tissue of the chest wall, so that the system is independently contained within the body.
As the heart produces its electrical signal, the pulse is transferred through the probe and back to the controller. The controller then uses this information to determine if the heart is beating properly. If not, the controller automatically informs either the demand pacer capacitor or defibrillator capacitor to transmit its respective pulse to its respective electrode. The pulses then travel through the blood and into the surrounding heart tissue, thereby defibrillating or demand pacing the heart. Finally, the charge leaves the body by the ground electrode.
One of the dilemmas associated with the above-described invention is that it is much easier for electricity to travel through blood than it is for electricity to travel through heart tissue. As a result, a majority of the defibrillation pulse, which is delivered in the blood, travels directly to the ground electrode through the blood, rather than entering the heart tissue. Accordingly, the heart is not defibrillated. Attempts have been made to resolve this problem by increasing the energy of the defibrillation pulse. Such an alternative, however, has additional drawbacks.
Blood is predominantly made of water. In turn, water molecules are made of hydrogen and oxygen. Passing a high electrical current through water breaks down the water molecules to form hydrogen and oxygen gas bubbles. This is a process known as electrolysis. Studies have found that excessively high defibrillation pulses can result in the electrolysis of the blood, thereby forming hydrogen and oxygen gas bubbles within the heart. Such gas bubbles can build up enough pressure within the heart to tear the heart tissue.
Furthermore, raising the strength of the defibrillation pulse increases the risk to the patient. If one defibrillation pulse should conduct better than another, an excessively high defibrillation pulse could result in damage to the heart tissue.
Finally, increasing the size of the defibrillation pulse requires a larger capacitor to deliver the pulse which in turm increases the size of the regulator. Also, use of a larger capacitor requires either a larger battery to be implanted or an increase in the frequency in which the battery must be replaced by implanting a new regulator. Such options increase the inconvenience to the patient.
Another troubling aspect is that it is difficult to target a specific charge with the previous defibrillator and demand pacer catheters. At times, it may be beneficial to direct a defibrillation or demand pacer pulse to a specific point in the heart. The previous catheters are free-floating within the heart. Therefore, they shift position with the movement of the patient or the beat of the heart. Hence, it is difficult to target the pulse.
Accordingly, some of the problems associated with the previous catheters used to defibrillate and demand pace the heart include: multiple and complex operations to attach the required electrodes, the necessity to use excessively high energy pulses that result in gas bubbles and increased threat to the patient, and the inability to strategically target a pulse for optimal effect.